netdev - Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel virtual address

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Date:   Thu, 13 Dec 2018 16:58:04 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] vhost: access vq metadata through kernel
 virtual address

On Thu, Dec 13, 2018 at 04:18:40PM -0500, Konrad Rzeszutek Wilk wrote:
> .giant snip..
> > > +	npinned = get_user_pages_fast(uaddr, npages, write, pages);
> > > +	if (npinned != npages)
> > > +		goto err;
> > > +
> > 
> > As I said I have doubts about the whole approach, but this
> > implementation in particular isn't a good idea
> > as it keeps the page around forever.
> > So no THP, no NUMA rebalancing, userspace-controlled
> > amount of memory locked up and not accounted for.
> > 
> > Don't get me wrong it's a great patch in an ideal world.
> > But then in an ideal world no barriers smap etc are necessary at all.
> 
> So .. suggestions on how this could be accepted? As in other ways
> where we still get vmap and the issues you mentioned are not troubling you?
> 
> Thanks!

I'd suggest leave vmap alone and find ways to speed up accesses
that can fault.

-- 
MST